“Where is this word, “trinity,” in the Bible? If its not in the Bible, why do you believe it?”
Because the scripture teaches it, and thus we sum up that teaching in the word “Trinity.”
Are you so ignorant that you don’t even know what the Bible teaches about the nature of God?
The concept of the Trinity as presently understood comes from Tradition, not Scripture. There are scriptural proof texts on which it is based, but the Trinity as we understand it comes from the Tradition of the Church.
Following Sola Scriptura alone regarding the Trinity has lead to many heresies such as Oneness Pentecostals. Scripture is the Heart of the Church. A Church without Scripture is dead. Scripture without the Church that collected and defined its canon leads to death too, as these groups that deny the Trinity based on a strict literal interpretation of scripture so aptly prove.